Radiation effects in high-temperature thermal diffusion measurements using the laser-flash method

A theoretical analysis has been made to account for radiation effects in thermal diffusivity measurements at high temperatures. The analysis considers the laser-flash method based on the three-layer technique for melts. The gray-body approximation is assumed to be valid for the liquid sample which is sandwiched between two metal layers. The analysis considers radiation from the upper and lower metallic plates to the surroundings and the radiative flux due to multiple reflections between the two plates. Finite pulse time effects and the absorption of radiation in the liquid sample are also considered. The model equations have been solved numerically using a finite difference scheme. Calculations have been made for a typical liquid sample representing an oxide melt in a platinum cell assembly in the temperature interval 1273–1673 K. The effects of parameters such as sample thickness, thermal diffusivity, effective absorptivity, etc. on the temperature transient of the back surface of the lower platinum layer have been investigated. The results indicate that the radiation flux at the top surface is relatively very high initially (time≪1 s). It shows a rapid decrease with respect to time and attains a small but finite value. The radiation flux from the bottom surface increases rapidly and approaches this finite value as expected. A comparison of the maximum temperature increase of the back surface of the lower platinum layer, with and without radiation correction at 1673 K, shows a typical difference of about 8%, which is quite significant. Calculations also show that liquid samples with effective absorption coefficients in the range zero to 100 m−1 can be considered to be completely transparent to radiation.